1. Field of the Invention
The invention relates to a process for industrial fractionation of polymers using an extraction column and an extraction agent as well as an apparatus for carrying out said process.
2. Description of the Prior Art
The fractionation of polymers from a solution is already known from M. J. R. Cantow: Polymer Fractionation; Academic Press N.Y. 1967, and from M. Hoffmann, H. Kromer, R. Kuhn: Polymer analysis, Thieme Verlag, Stuttgart, 1977. The polymer present in diluted solution is separated according to the molecular weight and/or according to the chemical composition with the aid of phase equilibria, and by frequent repetition of discontinuous basic steps and appropriate recombination of intermediate fractions good separating results can be achieved. However, an economic exploitation of the process is prevented by a high expenditure of working time and solvent required in a fractionation exceeding the gram scale.
A process of the type mentioned at the beginning is already disclosed in DE-OS 3,242,130. In this known process advantageous use is made of the effect that from polymer samples of high molecular and/or chemical irregularity products which are more uniform can be prepared if the polymers to be fractionated are dissolved in a suitable one or multi-component solvent and said solution or said feed extracted continuously in counter flow with the aid of a second solution or extraction agent containing the same solvent components as the feed. The solution component(s) in the feed and in the extraction agent is/are so chosen that firstly the entire system of starting polymers and solvent component(s) at the operating temperature has a miscibility gap, that secondly the composition of the phase in the phase diagram corresponds to a point outside the miscibility gap and that thirdly the extraction agent is made up so that the connecting line between the feed and extraction agent (operating line) intersects the miscibility gap.
To optimize the separation the ratio of the flows of feed and extraction agent was so chosen that the operating point, i.e. the mean composition of the total content of the fractionating apparatus in stationary operation, lies at higher polymer concentrations than the intersection point of the operating line with the respective portion of the separation curve belonging to the lower polymer concentration. In all cases the operating point is selected in the interior of the miscibility gap. The fractionation is achieved in that the polymer molecules introduced into the extraction column divide themselves up to a different extent amongst oppositely flowing phases in accordance with their molecular weight, their chemical composition or their geometric structure, the separation being facilitated by additional steps, for example a high dispersion of the phases. Thus, the operation is carried out with a particular form of a continuous counter-flow extraction in which preferably an "inner miscibility gap" is used in which a separation takes place only in the presence of the polymer. Put simply, for example the polymer molecules of different length must not decide whether they prefer a certain solvent, which is only limitedly miscible with another, to another solvent but merely whether they wish to be present in the more diluted or in the more concentrated phases flowing past each other.
A disadvantage in this known process is that for thermodynamic reasons it cannot be avoided that the sol removed still contains considerable proportions of long chains, i.e. that a satisfactory fractionation can only be achieved by several different experiments. The known fractionating process is therefore to be considered as technically impracticable.